1. Field of the Invention
The invention relates to a halogen-free extra-high-voltage cable for railway rolling stock which is excellent in mechanical characteristics, aging resistance, oil resistance, low-temperature characteristics, flame retardancy and safety.
2. Description of the Related Art
In high-speed railway vehicles which need to take high-voltage power therein from overhead lines via pantographs in order to increase travel speed, extra-high-voltage cables for railway rolling stock have been used.
In more detail, the extra-high-voltage cables for railway rolling stock are categorized into a pull-through cable for connecting a pantograph to a transformer placed under the floor of the vehicle and a vehicle-crossover cable connecting between vehicles to supply power.
This type of extra-high-voltage cable for railway rolling stock is required to have electric characteristics as well as mechanical characteristics against flaws or abrasion, environmental resistance such as aging resistance or oil resistance, and flame retardancy. In addition, excellent low-temperature characteristics are also required because of being used in also cold climates.
Accordingly, chloroprene rubber excellent in such characteristics is conventionally used as a sheath material to put such an extra-high-voltage cable into practical use.
However, the extra-high-voltage cable using chloroprene rubber is configured to produce a large amount of non-flammable halogen gas in the event of a fire so that combustion of the cable is prevented by insulating the cable from oxygen. Halogen gases, etc., produced in such occasion are often toxic even though sufficient flame retardancy is exhibited, which means that it is disadvantageous in that smoke containing poisonous gas is produced in a large amount.
Therefore, a halogen-free extra-high-voltage cable for railway rolling stock emphasizing fire safety has been desired, in which a metal hydroxide such as magnesium hydroxide or aluminum hydroxide is mixed as a flame retardant, in place of chloroprene rubber, with a polyolefin-based resin.
Since it is difficult to achieve a combination of sufficient flame retardancy and excellent mechanical characteristics only by use of metal hydroxide, methods in which a phosphorus-containing flame retardant such as red phosphorus is added as a flame-retardant aid (JP-A 2002-42574, JP-A 2002-42575 and JP-A 2003-113276) and methods in which 1,3,5-triazine derivative such as melamine cyanurate is added to ethylene-vinyl acetate copolymer (EVA) with a high vinyl acetate content as a base polymer (Japanese Patent Nos. 3632735, 4398552 and JP-A 2005-200574) have been disclosed in order to use high flame-retardant halogen-free resin compositions for wires and cables.
However, addition of phosphorus-containing flame retardant or 1,3,5-triazine derivative as a flame-retardant aid is not suitable for halogen-free extra-high-voltage cables for railway rolling stock which are required to have fire safety since toxic phosphine (PH3), nitrogen oxide or hydrogen cyanide are produced at the time of combustion even though sufficient flame retardancy is obtained.